Pentaquark and Tetraquark states
TL;DR: Recently, the LHCb collaboration not only confirmed the existence of the hidden-charm pentaquarks, but also provided strong evidence of the molecular picture as discussed by the authors, where the authors reviewed the experimental and theoretical efforts on the hidden heavy flavor multiquark systems in the past three years.
About: This article is published in Progress in Particle and Nuclear Physics.The article was published on 2019-03-28 and is currently open access. It has received 465 citations till now. The article focuses on the topics: Tetraquark & QCD sum rules.
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Technische Universität München1, Ludwig Maximilian University of Munich2, Budker Institute of Nuclear Physics3, Novosibirsk State University4, Russian Academy of Sciences5, Forschungszentrum Jülich6, Moscow Institute of Physics and Technology7, National Research Nuclear University MEPhI8, Fudan University9, Beihang University10, University of Cambridge11, Chinese Academy of Sciences12
TL;DR: A review of the progress in the field of exotic $XYZ$ hadrons can be found in this article, with a summary on future prospects and challenges, as well as a survey of the current state-of-the-art.
Abstract: The quark model was formulated in 1964 to classify mesons as bound states made of a quark-antiquark pair, and baryons as bound states made of three quarks. For a long time all known mesons and baryons could be classified within this scheme. Quantum Chromodynamics (QCD), however, in principle also allows the existence of more complex structures, generically called exotic hadrons or simply exotics. These include four-quark hadrons (tetraquarks and hadronic molecules), five-quark hadrons (pentaquarks) and states with active gluonic degrees of freedom (hybrids), and even states of pure glue (glueballs). Exotic hadrons have been systematically searched for in numerous experiments for many years. Remarkably, in the past fifteen years, many new hadrons that do not exhibit the expected properties of ordinary (not exotic) hadrons have been discovered in the quarkonium spectrum. These hadrons are collectively known as $XYZ$ states. Some of them, like the charged states, are undoubtedly exotic. Parallel to the experimental progress, the last decades have also witnessed an enormous theoretical effort to reach a theoretical understanding of the $XYZ$ states. Theoretical approaches include not only phenomenological extensions of the quark model to exotics, but also modern non-relativistic effective field theories and lattice QCD calculations. The present work aims at reviewing the rapid progress in the field of exotic $XYZ$ hadrons over the past few years both in experiments and theory. It concludes with a summary on future prospects and challenges.
298 citations
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Ludwig Maximilian University of Munich1, Technische Universität München2, Budker Institute of Nuclear Physics3, Russian Academy of Sciences4, Novosibirsk State University5, Forschungszentrum Jülich6, National Research Nuclear University MEPhI7, Moscow Institute of Physics and Technology8, Fudan University9, Beihang University10, University of Cambridge11, Chinese Academy of Sciences12
TL;DR: A review of the progress in the field of exotic X Y Y Z hadrons can be found in this paper, where the authors present a summary of the future prospects and challenges.
207 citations
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TL;DR: In this paper, the invariant mass spectrum of the LHCb collision data at center-of-mass energies of 7, $8, $13, and $13\mathrm{\,TeV} was studied.
Abstract: Using proton-proton collision data at centre-of-mass energies of $\sqrt{s} = 7$, $8$ and $13\mathrm{\,TeV}$ recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of $9\mathrm{\,fb}^{-1}$, the invariant mass spectrum of $J/\psi$ pairs is studied. A narrow structure around $6.9\mathrm{\,GeV/}c^2$ matching the lineshape of a resonance and a broad structure just above twice the $J/\psi$ mass are observed. The deviation of the data from nonresonant $J/\psi$-pair production is above five standard deviations in the mass region between $6.2$ and $7.4\mathrm{\,GeV/}c^2$, covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow $X(6900)$ structure are measured assuming a Breit--Wigner lineshape.
204 citations
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South China Normal University1, Université Paris-Saclay2, Chinese Academy of Sciences3, Nankai University4, Xinyang Normal University5, Guangzhou University6, Hunan University7, Nanjing University8, Huazhong University of Science and Technology9, fondazione bruno kessler10, Peking University11, Hunan Normal University12, Nanjing Normal University13, Northeastern University (China)14, Southwest Jiaotong University15, University of Zagreb16, Joint Institute for Nuclear Research17, Zhengzhou University18, Lanzhou University19, Shandong University20, Central China Normal University21, Southeast University22, Fudan University23, Chongqing University24, VU University Amsterdam25, Nanjing University of Aeronautics and Astronautics26, Dalian University of Technology27, Beihang University28, Shanghai Jiao Tong University29, Lanzhou University of Technology30, University of Science and Technology of China31, The Chinese University of Hong Kong32, Nanjing University of Posts and Telecommunications33, Huangshan University34, University of Electronic Science and Technology of China35, Tsinghua University36, Michigan State University37, Beijing Normal University38, Sun Yat-sen University39, Wuhan University40, China University of Geosciences (Wuhan)41
TL;DR: In this article, an Electron-ion collider in China (EicC) has been proposed, which will be constructed based on an upgraded heavy-ion accelerator, High Intensity heavy ion Accelerator Facility (HIAF), together with a new electron ring.
Abstract: Lepton scattering is an established ideal tool for studying inner structure of small particles such as nucleons as well as nuclei. As a future high energy nuclear physics project, an Electron-ion collider in China (EicC) has been proposed. It will be constructed based on an upgraded heavy-ion accelerator, High Intensity heavy-ion Accelerator Facility (HIAF) which is currently under construction, together with a new electron ring. The proposed collider will provide highly polarized electrons (with a polarization of ∼80%) and protons (with a polarization of ∼70%) with variable center of mass energies from 15 to 20 GeV and the luminosity of (2–3) × 10$^{33}$ cm$^{−2}$ · s$^{−1}$. Polarized deuterons and Helium-3, as well as unpolarized ion beams from Carbon to Uranium, will be also available at the EicC.The main foci of the EicC will be precision measurements of the structure of the nucleon in the sea quark region, including 3D tomography of nucleon; the partonic structure of nuclei and the parton interaction with the nuclear environment; the exotic states, especially those with heavy flavor quark contents. In addition, issues fundamental to understanding the origin of mass could be addressed by measurements of heavy quarkonia near-threshold production at the EicC. In order to achieve the above-mentioned physics goals, a hermetical detector system will be constructed with cutting-edge technologies.This document is the result of collective contributions and valuable inputs from experts across the globe. The EicC physics program complements the ongoing scientific programs at the Jefferson Laboratory and the future EIC project in the United States. The success of this project will also advance both nuclear and particle physics as well as accelerator and detector technology in China.[graphic not available: see fulltext]
154 citations
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TL;DR: In this paper, a review of threshold cusps and various triangle singularities in hadronic reactions, paying attention to their manifestations in phenomena related to exotic hadron candidates is presented.
113 citations
References
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TL;DR: The complete review as discussed by the authors is published online on the website of the Particle Data Group (http://pdg.lbl.gov) and in a journal. Volume 1 is available in print as thePDG Book.
Abstract: The complete Review(both volumes) is published online on the website of the Particle Data Group(http://pdg.lbl.gov) and in a journal. Volume 1 is available in print as thePDG Book. AParticle Physics Bookletwith the Summary Tables and essential tables, figures, and equations from selected review articles is also available.
6,464 citations
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TL;DR: In this paper, a systematic study is made of the non-perturbative effects in quantum chromodynamics, where the basic object is the two-point functions of various currents and the terms of this series are shown to be of two distinct types.
3,874 citations
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TL;DR: The three quark system is studied in a relativized version of the quark potential model with chromodynamics and the spectrum of baryons is described with parameters consistent with those of an analogous study of meson spectroscopy.
Abstract: We have studied the three quark system in a relativized version of the quark potential model with chromodynamics. With parameters consistent with those of an analogous study of meson spectroscopy we obtain a successful description of the spectrum of baryons. The model naturally explains the apparent absence of spin‐orbit interactions in baryons.
2,203 citations
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TL;DR: In this paper, a new mathematical framework for the Wess-Zumino chiral effective action is described, and it is shown that this action obeys an a priori quantization law, analogous to Dirac's quantization of magnetic change.
2,132 citations
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TL;DR: In this article, the existing results concerning mesons and glue states in the large-N limit of QCD are reviewed, and it is shown how to fit baryons into this picture.
1,934 citations